To provide cellular wireless communication service, a wireless service provider typically operates a radio access network (RAN) that defines one or more wireless coverage areas in which user devices can be served by the RAN and can thereby obtain connectivity to broader networks such as the public switched telephone network (PSTN) and the Internet.
A typical RAN may include one or more base transceiver stations (BTSs) (e.g., macro network cell towers and/or femtocells), each of which may radiate to define a wireless coverage areas such as cells and cell sectors in which WCDs can operate. Further, the RAN may include one or more radio network controllers (RNCs) or the like, which may be integrated with or otherwise in communication with the BTSs, and which may include or be in communication with a switch or gateway that provides connectivity with one or more transport networks. Conveniently with this arrangement, a cell phone, personal digital assistant, wirelessly equipped computer, or other user device (whether or not actually operated by a user) that is positioned within coverage of the RAN can then communicate with a BTS and in turn, via the BTS, with other served devices or with other entities on the transport network.
In general, a RAN will communicate with served user devices according to an agreed air interface protocol, examples of which include CDMA, iDEN, WiMAX, LTE, TDMA, AMPS, GSM, GPRS, UMTS, or EDGE, and others now known or later developed. The air interface protocol will define a mechanism to distinguish communications in one coverage area from those in adjacent coverage areas and to distinguish between communications within a given coverage area. For instance, under the CDMA protocol, each sector has a unique “PN offset” that is used to encode communications carried out in the sector in a manner that distinguishes communications carried in adjacent sectors. Further, each sector defines various control channels and traffic channels, each encoded with a respective “Walsh code”. Other examples are possible as well.
A RAN will typically broadcast a pilot signal or the like respectively in each coverage area, to enable user devices to detect and evaluate cellular coverage in that area. For instance, when a user device is operating on an assigned traffic channel in a given coverage area, the user device may regularly monitor the strength (e.g., signal-to-noise ratio (SNR)) of the pilot signal in that coverage area and may likewise monitor the strength of pilot signals that may come from nearby coverage areas. If the pilot signal from an adjacent coverage area becomes sufficiently stronger than the pilot signal in the current coverage area (e.g., as a result of the user device moving toward the adjacent coverage area), the user device may then engage in control channel signaling with the RAN to arrange for a handoff from the current coverage area to the adjacent coverage area.
Under CDMA, and certain other air interface protocols, a user device can operate in more than one wireless coverage area at a time. This is particularly beneficial when a user device enters or passes through an area of overlap between two or more coverage areas, as the user device may then engage in a “soft handoff” process, in which the device communicates with both coverage areas at the same time, thus facilitating a “make before break” transition from one coverage area to another. For instance, if the user device is operating on an assigned traffic channel in coverage area A and then enters into an area of overlap between coverage areas A and B, the user device may signal to the RAN and the RAN may assign the user device a traffic channel in coverage area B, while allowing the user device to retain the assigned traffic channel in coverage area A. In this state, the user device would then communicate with the RAN concurrently through both coverage area A and coverage area B. In turn, if the user device then moves into coverage area B and out of coverage area A, the user device may engage in control channel signaling with the RAN and the RAN may release the traffic channel that was assigned to the user device in coverage area A.
To facilitate soft handoff, a user device will typically maintain in its memory an “active set” that lists the coverage areas (or “connection legs”) in which the user device has assigned traffic channels, and the RAN will likewise maintain a record of the user device's active set and will communicate with the user device in each listed coverage area.
Thus, in practice, if the user device is operating in just coverage area A and then moves into an area of overlap of coverage area A and coverage area B, the user device may send a signaling message to the RAN that informs the RAN of a detected pilot signal strength for coverage area B. If the RAN agrees to allow a handoff, the RAN may then assign the user device a traffic channel in coverage area B, update the RAN's record of the user device's active set to specify coverage areas A and B, and send a handoff directive to the user device instructing the user device to operate on traffic channels in coverage areas A and B.
Similarly, if the user device is operating in coverage areas A and B and then moves into an area of overlap with coverage area C, the user device may send a signaling message to the RAN that informs the RAN of a detected pilot signal strength for coverage area C. If the RAN agrees to allow a further handoff, the RAN then assign the user device a traffic channel in coverage area C, update the RAN's record of the user device's active set to specify coverage areas A, B, and C, and send a handoff directive to the user device instructing the user device to operate on traffic channels in coverage areas A, B, and C. A similar process may of course occur as well when the user device leaves a coverage area.
In addition, as a user device operates on an assigned traffic channel in any given coverage area, the user device and RAN may typically engage in a power control process to control the transmission power used for communication on that traffic channel. Optimally, this power control process will help to keep the traffic channel communication strong enough to overcome interference stemming from other communications in the coverage area and from topographical obstructions, and will help to keep the traffic channel communication from becoming so strong that it would unduly interfere with communications by other user devices.
By way of example, the user device may regularly monitor the SNR of traffic channel communications that the user device receives from the RAN and compare the SNR to a power control setpoint. If the SNR is lower than the power control setpoint, then the user device may send to the RAN a power control command directing the RAN to increase the RAN's transmission power in an effort to improve the SNR. Whereas, if the SNR is higher than the power control setpoint, then the user device may send to the RAN a power control command directing the RAN to decrease the RAN's transmission power in an effort to avoid needlessly strong transmission. At the same time, the user device may also monitor an error rate (e.g., frame error rare) of the received signal and compare the error rate to a target error rate value. If the error rate is higher than the target, that may indicate that the power control setpoint is too low, and so the user device may increase the power control setpoint. Whereas, if the error rate is lower than the target, that may indicate that the power control setpoint is needlessly high, and so the user device may decrease the power control setpoint.